I'd like the respondent to check exactly two of the three options, preventing him/her the possibility to check only one option or the whole lot.
For as much as I've tried I couldn't manage to solve this, even trying other options such as arrays of various configurations.
In the "logic" section, as it were, there is the possibility to set a minimum and a maximum of answers, but this merely results in an a recommendation message that invites the respondent to feed two answers only, and becomes green when the precisely required number of answers is given, non preventing, however, an undesired number of answers.
Thank you in advance for any help.

What? Just simple as that! I was expecting dozens of js lines and a fair measure of special effects.
It's deceiving (... that I did't make out so simple a brilliant solution).
Already implemented with immense satisfaction.
Should you happen to drop in Rome (a most likely event sooner or later) do have a coffee with me.

thank you for the suggestion.
However, that's precisely what I did at first run, setting max and min answers to "2".
Unfortunately the difference consisted only in a message, up left under the question, recommending the respondent to check only two answers, changing the colour to green as a confirmation of a correct answer, setting to a different colour in case the answer was limited to one check or to three.

Here again I confess that time is dragging me to ask frantically for help whereas a more attentive study of the manual should be the rule. That much apology is due for fairness to all those responding with suggestions, indications and outright solutions. first of all the LS staff.